1. Field of Invention
The present invention relates to a non-linear optical material. Particularly, the present invention relates to a non-linear optical material comprising a bismuth film that exhibits strong non-linear optical effects including non-linear refraction and non-linear absorption. When an incident beam passes through the bismuth film, the refractive index and the absorption coefficient of the bismuth film change remarkably with the intensity of the incident beam.
2. Description of Related Art
The refractive index xe2x80x9cnxe2x80x9d and the absorption coefficient xe2x80x9cxcex1xe2x80x9d of a common optical material usually change very slightly with the intensity of an incident beam, and can be taken as two constants, n0 and xcex10, respectively. Such an optical material is called a linear optical material. For a non-linear optical material, however, the refractive index n and the absorption coefficient xcex1 change with the intensity of the incident beam. If the variation of the refractive index n of a non-linear optical material is directly proportional to one order of the intensity I of an incident beam, which is expressed by n=n0+n1I, the non-linear refractive effect becomes stronger as the intensity of the incident beam increases. Therefore, when a laser beam is conducted through the non-linear material, a self-focusing effect or a self-defocusing effect is produced, wherein the non-linear material acts as a convex lens or as a concave lens (See Y. R. Shen, xe2x80x9cThe Principles of Non-linear Opticsxe2x80x9d, John Wiely and Sons, 1984, p. 303xcx9c331).
Moreover, if the variation of the absorption coefficient xcex1 of a non-linear optical material is directly proportional to one order of the intensity I of an incident beam, the absorption coefficient a can be expressed by xcex1=xcex10+xcex11I.
The properties of a non-linear optical material can be easily measured by a method described by M. Sheik-Bahae et al. in IEEE. J. Quantum Electron, 26, p. 760xcx9c769 (1990). The method is called xe2x80x9cZ-SCANxe2x80x9d and can be used to precisely measure the variations of the refractive index and the absorption coefficient of a non-linear optical material.
For common materials, the values of the first-order terms n1 and xcex11 both are quite small. For example, the n1 value of glass is 4xc3x9710xe2x88x927 cm2/GW (See W. Koechner, xe2x80x9cSolid-State Laser Engineeringxe2x80x9d, Springer, 1999, p. 672), a very intense beam is thus needed to induce a non-linear refractive effect. Specifically, when the refractive index of glass is increased by only one millionth, the intensity I of the incident beam must be as high as 2.5 GW/cm2. Therefore, in order to be possible to use a weaker incident beam and thereby to facilitate the applications of non-linear optical mechanisms, finding a non-linear optical material with larger n1 and xcex11 values is important.
Accordingly, the present invention provides a non-linear optical material comprising a bismuth film that exhibits strong non-linear effects with the intensity of an incident beam. The bismuth film is thin enough to be semi-transparent for an incident beam and the non-linear optical effects include non-linear refraction and non-linear absorption that are mainly attributed to the bismuth film.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.